Automated inspection and review systems are important in process control and yield management for the semiconductor and related microelectronics industries. Such inspection and review systems may include optical and electron-beam based systems.
Semiconductor chip manufacturing is very complex due to the fact that it involves hundreds of process steps before the final device is made. The number of process steps required to manufacture the chip is increasing dramatically as associated design rules shrink. During manufacturing of semiconductor devices, detection of physical defects and electrical failures earlier in the fabrication process has become increasingly important in order to shorten product development cycles and increase product yield and productivity. Automated inspection and review systems are used to capture yield killing defects to discover the cause of the yield loss. Electron beam inspection and review systems provide unparalleled sensitivity for small defects because of the very high resolutions of electron beam systems compared to their optical counterparts.
In a conventional scanning electron microscope, a beam of electrons is scanned over a sample (e.g., a semiconductor wafer). Multiple raster scans are typically performed over an area of the sample. The beam of electrons either interact with the sample and cause an emission of secondary electrons or bounce off the sample as backscattered electrons. The secondary electrons and/or backscattered electrons are then detected by a detector that is coupled to a computer system. The computer system generates an image that is stored and/or displayed on the computer system.
Typically, a certain amount of charge is required to provide a satisfactory image. This quantity of charge helps bring out the contrast features of the sample. Although conventional electron microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications. For example, in the case of a sample made of a substantially insulating material (e.g., silicon dioxide), performing one or more scans over a small area causes the sample to accumulate excess positive or negative charge in the small area relative to the rest of the sample. Samples having metal structures disposed on an insulating substrate with no conduction path to ground (floating metal) tend to charge up to large voltages, which can affect the primary electron beam used for scanning the sample. The stray electric fields due to local charge build up could be extremely high on floating metal structures since the charge tends to accumulate near the external surface of the floating metal and these structures often have sharp edges. Often this results in blurry images due to defocus and/or astigmatism and can result in undesired beam position errors on the sample, making it very difficult to acquire multi-frame images. The charge that is built up during the scanning process can persist for a prolonged period of time and can cause distortion in subsequent runs.
The existing automatic methods to correct defocus, astigmatism and beam position errors induced by surface charging are highly unreliable and tend to be slow due to the dynamics of the charging. Therefore, it would be desirable to provide a method and system that cure the shortcomings of the prior approaches as identified above.